1. Field of the Invention
The present invention relates to an organic electroluminescence element. In particular, the invention relates to an organic electroluminescence element with high light-emission efficiency and excellent durability.
2. Description of the Related Art
An organic electroluminescence element (hereinafter, referred to as an “organic EL element” in some cases) is composed of a light-emitting layer or a plurality of organic functional layers containing a light-emitting layer, and a pair of electrodes sandwiching these layers. The organic EL element is a device for obtaining luminescence by utilizing luminescence from excitons each of which is obtained by recombining an electron injected from a cathode with a hole injected from an anode to produce the exciton in the light-emitting layer, or luminescence from excitons of other molecules produced by energy transmission from the above-described excitons.
Heretofore, an organic EL element has been developed by using a laminate structure from integrated layers in which each layer is functionally differentiated, whereby brightness and element efficiency are remarkably improved. For example, a two-layer laminated type element obtained by laminating a hole transport layer and a light-emitting layer which also functions as an electron transport layer; a three-layer laminated type element obtained by laminating a hole transport layer, a light-emitting layer, and an electron transport layer; and a four-layer laminated type element obtained by laminating a hole transport layer, a light-emitting layer, a hole blocking layer, and an electron transport layer have been frequently used (see, for example, “Science”, vol. 267, No. 3, page 1332, (1995)).
However, many problems still remain for putting organic EL elements to practical use. First, there is a need to attain high light-emission efficiency, and second, there is a need to attain high drive durability. In particular, deterioration in quality during continuous driving is a most significant problem.
For instance, Japanese Patent Application Laid-Open (JP-A) No. 2002-324680 proposes an attempt to dispose a region containing a compound that constitutes a light-emitting layer and a compound that constitutes an electron transport layer between the light-emitting layer and the electron transport layer, so as to lower a barrier when electrons flow from the electron transport layer to the light-emitting layer. It is disclosed in the disclosure to be preferable that, in this region, a concentration of the compound that constitutes the light-emitting layer increases toward the light-emitting layer so as to lower the barrier. However, although such means may lower the carrier transfer resistance of an adjacent layer, these are not sufficient means for attaining high light-emission efficiency and high drive durability.
Also, in the case where an organic EL element has a laminate structure, carrier injectability is reduced due to a barrier between the respective layers, and there are problems of an increase in driving voltage and a reduction in durability. As a means for reducing such a barrier between the respective layers, for example, JP-A No. 2002-313583 proposes to provide a gradation in the concentration, in the respective layers, of a hole injection material, an electron injection material, a hole transport material or an electron transport material contained in the respective layers. In such a configuration, a light-emitting material in a light-emitting layer is arranged at a definite concentration in a restricted region in the light-emitting layer which is formed of a bipolar mixed layer. Even in this configuration, as the light emission may take place only in the restricted region within a region where the light-emitting material is arranged, this is not sufficient as a means to achieve high light-emission efficiency.
In designing a practically useful organic EL element, it is an extremely important problem to make high external quantum efficiency and high drive durability compatible with each other. This problem is a problem which is always required to be improved.